Containers for chemically sensitive materials such as food products traditionally have been made from inorganic materials such as glass. Glass containers are transparent and permit the consumer to view the product before purchasing it. Moreover, glass containers are essentially impermeable to atmospheric gases such as oxygen and hence protect the product. However, glass containers are expensive, heavy and susceptible to breakage. Accordingly, considerable effort has been devoted to development of containers made from polymeric materials such as thermoplastics. Thermoplastic containers can be made inexpensively. They are light in weight and hence inexpensive to ship. They are resistant to breakage and can be fabricated in convenient shapes.
However, polymeric containers ordinarily are permeable to atmospheric gases and to gases in the packaged product. This drawback has limited the use of polymeric containers in many applications. Various approaches have been taken towards eliminating the permeability of polymeric containers. Certain polymers have relatively low permeability to particular gases. Containers fabricated from these polymers sometimes can provide satisfactory resistance to permeation for particular applications. However, use of these particular, low permeability polymers can introduce additional problems of cost, transparency, or strength. In certain cases, the low permeability polymers are incompatible with the product to be contained. To alleviate these drawbacks composite containers incorporating one or more layers of a low permeability polymer in conjunction with layers of other polymers have been used. This approach is costly and can make it more difficult to recycle the containers using common recycling techniques such as melt processing.
Various proposals have been advanced for rendering polymeric materials less permeable to oxygen and other gases by depositing thin films incorporating inorganic materials such as oxides of silicon on a substrate consisting of the polymeric material. Such deposition coating is enhanced by applying an electrical potential to the substrate being coated. For example, an acceptable barrier coating can be formed on flat substrates by placing such flat substrates on an electrically conductive plate and electrically coupling the plate to a direct current, audio frequency or radio frequency power supply, while reactive oxides and organosilicon gas react in the vicinity of the substrate.
In Applicant's copending commonly assigned U.S. patent application, Ser. No. 07/889,637, entitled METHODS AND APPARATUS FOR DEPOSITING BARRIER COATINGS, filed with the U.S. Patent and Trademark Office on May 28, 1992, a method and apparatus for depositing barrier coatings on polymeric substrates such as containers is disclosed. The copending application teaches the conversion of an oxidizing gas into plasma in a plasma zone remote from a treatment chamber. The resulting plasma-activated oxidizing species may be delivered to the vicinity of a thermoplastic container. An organosilicon reactant vapor is separately and simultaneously delivered to the vicinity of the container so that the organosilicon vapor and oxidizing active species mix in proximity to the container where they react. The products of the reaction are deposited on the container. The copending application further discloses the use of an electric field which is applied to the container, so that the reaction products are deposited under the influence of the electric field to form the barrier coating. In this process, the coated surface is treated by electrically charged species in the gaseous reaction mix. In certain preferred embodiments, the coating is deposited on the inside of the container, and a biasing voltage is applied to the outside of the container by an electrically conductive shell which surrounds the container.
In other processes, surfaces can be treated with electrically charged species for purposes other than coating. Merely by way of example, such processes may be used for surface activation, cleaning, polymer grafting and other tasks. Where these and other electric-field-assisted treatment processes are employed to treat the outside of a container, the biasing voltage should be applied from inside the container. However, it has been difficult heretofore to uniformly bias the wall of a rigid container by uniformly applying an electric charge to the inner surface of the container. Prior efforts to apply a uniform electric charge to the inner surface of a container, especially a narrow-neck container, using mechanical means such as metal objects in the container have encountered drawbacks.
For example, Mackowski, U.S. Pat. No. 4,746,538, discloses the use of a radially expandable cathode disposed within a glass bottle to aid in the coating of the exterior of same. Such devices can be fragile and slow to operate, and can contaminate the interior of the container.
Despite the efforts disclosed in the aforementioned prior art reference, as well as other substantial efforts in the art, there are needs for improved methods and apparatus for uniformly applying an electrical bias from the inside of a container during a treatment processes. The present invention fulfills these needs.